Because of its juxtaposition to blood, the cerebral endothelium (which forms the blood-brain barrier, BBB) has been thought to be relatively resistant to the effects of cerebral ischemia. However, examination of taurine, glutamine and myo-inositol influx into brain (all Na+-dependent processes) indicate a marked early (<1 hour) reduction in transport during focal cerebral ischemia suggesting that endothelial cell injury could play a role in primary, rather than secondary, ischemic brain damage. This may be particularly the case if efflux from as well as influx into brain are affected since those efflux systems are involved in controlling the concentration of potentially toxic factors in the brain extracelluar space. This proposal, therefore, has two major goals: to determine whether energy-dependent efflux from brain to blood is inhibited during cerebral ischemia (Specific Aims 1 and 2) and to examine whether changes in influx and efflux transport mechanisms at the blood-brain barrier contribute to ischemic brain damage (Specific Aim 3). The cerebral volume of distribution reached by [3H] vinblastine (a P-glycoprotein substrate) and p-[3H] aminohippuric acid (PAH, an organic acid transporter substrate) will be determined following middle cerebral artery occlusion in rat and mouse (Specific Aim 1). Whether an increased volume of distribution with ischemia reflects a change in influx or an alteration in efflux at the blood-brain barrier will then be determined, the latter by examining the effect of cerebral ischemia in the absence of BBB P-glycoprotein (the mdr la knock out mouse) or during probenecid-induced inhibition of organic acid transport. Specific Aim 2 will examine the mechanism by which ischemia inhibits efflux, by examining PAH, L-glutamate and methyl aminosobutyric acid efflux (an A-system amino acid transporter substrate) uptake into choroid plexus using ventriculo-cisternal perfusion. Specific Aim 3 will determine the effect of altering specific transporters at the BBB on ischemic brain injury and will examine whether drugs known to ameliorate the effect of reperfusion on blood-brain barrier disruption actually have their effects by altering transport during ischemia. Determining whether early BBB dysfunction should be an alternate therapeutic target early during cerebral ischemia, the finding that there is an inhibition of energy-dependent efflux at the BBB during ischemia has major implications for drug delivery to the injured brain. P-glycoprotein and the organic acid transporter both play a major role in limiting the access of some drugs to the brain.